1. Field of the Invention
The present invention relates generally to page based holographic storage systems, and more particularly, to data retrieval from a holographic storage medium.
2. Related Art
In a common design for page-oriented holographic data storage systems, a coherent beam of light modulated with data is interfered with a plane-wave reference beam and the resulting interference pattern is recorded in a holographic medium. This recorded pattern can be read out with an identical or phase conjugate plane-wave reference beam to reconstruct the data beam and recover the stored data.
Holographic media incorporating photopolymers show great potential for commercial use due to their large index contrast, high photosensitivity and low cost. One potential drawback with these materials is that their coefficient of thermal expansion and their index of refraction may change with temperature such that, in many holographic system designs, normal temperature changes may result in difficulty in recovering an entire data page using a single exposure.
Further, in holographic storage systems the tolerance of the reference beam's orientation may be inversely related to the thickness of the holographic medium. Because it is often desirable to use a holographic storage medium to store large amounts of data, it must be correspondingly thick, and there may be little tolerance in the reference beam's orientation. Additionally, in a commercial holographic storage system, the holographic medium will be regularly removed and replaced in the system, potentially resulting in angular errors in the direction and azimuth of the reference beam orientation. Angular misalignments due to rotation or tilt of the holographic medium make it difficult to recover an entire data page without compensating for the misalignment. Further, due at least in part to the thickness of a typical holographic storage medium, angular misalignments of less than 1 degree may render it difficult, if not impossible, to recover a data page in a single exposure in holographic storage systems.
Accordingly, there is a need for methods and systems for compensating for temperature and/or alignment changes in holographic storage systems.